Electrically-operated dispensing modules have been developed for product assembly lines requiring precise intermittent placement of small amounts of a viscous liquid, such as heated liquid adhesives, at a high speed onto a substrate moving past the liquid dispenser. Generally, an electrically-operated dispensing module includes a magnetic pole piece, a magnetic armature movable relative to the pole piece, a valve stem coupled for movement with the armature, and an electromagnetic coil. The armature is moved relative to the pole piece by selectively energizing and de-energizing the electromagnetic coil. When energized to initiate a dispensing cycle, an electromagnetic field produced by the electromagnetic coil magnetizes the armature and pole piece. The resulting movement of the armature toward the pole piece disengages or unseats the valve stem from the valve seat and opens the dispensing module. When the electromagnetic coil is de-energized, a return spring biases the armature away from the pole piece and this urges the valve stem into contact with the valve seat to close the dispensing module.
The electrically-operated dispensing module is cycled periodically between opened and closed positions to initiate and interrupt fluid flow for dispensing small, discrete volumes a substrate. Depending upon the cycle duration, the small volumes of viscous liquid may be dispensed as spaced-apart, substantially-round dots or as a line of spaced-apart beads. The cycle rate of the dispensing module can define the size and shape of dispensed dots and the characteristics of the leading and trailing edges of dispensed beads.
Conventional electrically-operated dispensing modules include one or more flux elements that strengthen the magnetic field by reducing or preventing magnetic flux loss. These flux elements are typically tubular structures that surround the armature, the electromagnetic coil, and the pole piece. Because the flux elements are formed from an electrically conductive material, the electromagnetic field generated by the electromagnetic coil induces eddy currents that produce electrical currents extending circumferentially about the flux elements. Such circumferential currents retard the dissipation of the magnetic field after the electromagnetic coil is de-energized.
The electromagnetic coil, armature, pole piece and flux elements of conventional electrically-operated dispensing modules participate in forming a magnetic circuit. A typical magnetic circuit incorporates an air gap that dissipates residual magnetism remaining in the magnetic circuit after the electromagnetic coil is de-energized. When conventional electrically-operated dispensing modules are opened, the pole piece and the armature are contacting and are not separated by an air gap. Because the air gap is absent, demagnetization of the pole piece and armature is slowed after the electromagnetic coil is de-energized and residual magnetism tends to hold the armature stationary against the pole piece.
An inability to abruptly remove the attractive force acting between the armature and the pole piece, such as due to the effects of circumferential electrical currents in flux elements and contact between the armature and pole piece, significantly lengthens the time required to shut off the dispensing module. As a result, conventional electrically-operated dispensing modules may not operate at frequencies high enough for certain applications. Also, successive dots tend to grow larger and successive beads tend to lengthen for later-occurring dispensing cycles due to lengthened shut off time.
Most dispensing modules include a removable nozzle containing a discharge passageway from which the small volumes of viscous liquid are dispensed. Usually, the nozzle is not directly heated but, instead, is heated by conduction with adjacent portions of the body of the dispensing module to which the nozzle is attached. The heated portions of the dispensing module are operated at a temperature appropriate for maintaining the viscous liquid at a desired temperature and viscosity without charring or otherwise degrading the physical properties of the viscous liquid. To promote efficient heat transfer to the nozzle, the adjacent module body portion should be formed from a material having high thermal conductivity.
Conventional electrically-operated dispensing modules incorporate an armature guide sleeve, typically formed of a non-magnetic stainless steel, that guides the reciprocating movement of the armature relative to the pole piece. Because of the arrangement of the nozzle and armature within the dispensing module, the guide sleeve is joined with the portion of the module body that is adjacent to the nozzle. The material forming the armature guide sleeve must be compatible with being joined by a process, such as welding or brazing, with the material forming the adjacent module body portion. Therefore, the selection of a non-magnetic stainless steel for forming the armature guide sleeve constrains the material selection for the module body portion to which it is attached, as dissimilar materials are often difficult to join.
A module body portion of stainless steel is readily joined with stainless steel guide sleeves, but stainless steel is a poor heat conductor. A module body portion of brass would promote heat transfer but cannot be joined with a stainless steel armature guide sleeve. Inefficient heat transfer through and from the adjacent module body portion reduces the temperature of the nozzle below the operating temperature of the dispensing module. Increasing the temperature of the dispensing module to increase the temperature of the nozzle is not feasible as the physical properties of viscous liquid within the liquid passageways of the dispensing module would be degraded. The temperature reduction increases the liquid viscosity in the nozzle such that viscous liquid dispensed from the discharge passageway may have a persistent string or tail of viscous liquid. These strings or tails of liquid adversely affect the appearance and/or quality of the finished product. As the substrate moves away from the dispensing module, the strings may also break, become airborne, and land on surrounding equipment, etc. This results in increased maintenance costs and cleaning, as well as potential equipment downtime.
What is needed, therefore, is an electrically-operated dispensing module capable of operating at high frequencies while maintaining reproducibility and accuracy among successive dispensed volumes of viscous liquid and preventing stringing or tailing.